Electromagnetic transducer



WQ J. MULDOON ELECTROMAGNETIC TRANSDUCER Sept. 26,1950

Filed May 17,- 1946 //v vE/v TOR ATTORNEYS Patented Sept. 26, 1950 UNITED STATES PATENT OFFICE ELECTROMAGNETIC TRANSDUCER William J. Muldoon, Garden City, N. Y., assignor,

by mesne assignments, to The Wheeler Insulated Wire Company, Incorporated, Bridgeport, Conn., a corporation of Delaware Application May 17, 1946, Serial No. 670,585

2 Claims. I

The present invention relates to transducers and is more particularly concerned with telephone transducers used in sound-powered or bat teryless systems, although the invention may advantageously be used in battery telephone systems as will hereinafter appear.

Transducers for use in sound-powered systems are usually of the magnetic armature type as distinguished from the magnetic diaphragm type. The magnetic armature type consists of an armature connected with a diaphragm. The armature may be of the balanced or of the reed type, but in either case the armature passes between two air gaps one at each end. The unit is provided with a permanent magnet or magnets for the polarizing flux, which passes through the two gaps in parallel, while the alternating signal fiux passes through two air gaps in series.

The principal object of the present invention is to provide a transducer of the magnetic armature type which is of simpler and more inexpensive construction, and which is of higher magnetic and electro-accoustic efficiency than existing transducers of this general class.

According to the present invention, the armature is clamped at one end between two permanent magnets. The magnets are provided with pole pieces terminating adjacent to the free end of the armature. The construction is such that the alternating flux is required to pass through only a single air gap. If the reluctances of all parts of the magnetic circuit other than the air gap were negligible, the unit would produce twice the force on the armature for any given signal current, as compared with the conventional reed type. Actually, the alternating flux is required to pass through the polarizing magnets for which the reluctance is not negligible, and the figure of merit is somewhat less than twice that of the conventional reed type, but with existing magnet materials of high energy product, it is possible to obtain a materially higher efiiciency than with the usual forms of reed transducers.

In the accompanying drawings illustrating what is now considered the preferred form of the invention, Fig. 1 is a sectional elevation; Fig. 2 is a plan view; Fig. 3 is an end view; and Fig. 4 is a diagram of the analogous electrical circuit.

As shown in the drawings, the unit comprises a base 4 to which is clamped a diaphragm 6. Mounted on the base is a magnetic structure including two magnetically equivalent permanent magnets 8 and I magnetized in series relation, that is, with the north pole of one adjacent the south pole of the other. Pole pieces I2 and I4 extend symmetrically from the ends of the magnets toward the center of the unit. An armature IB is clamped at one end at the magnetic center between the two magnets. The free end of the armature is normally centralized in the space between the ends of the pole pieces, whereby air gaps l8 and 20 are formed between the armature and the respective pole pieces. A coil 22 of conventional type is mounted within the pole pieces and surrounds the armature. A drive rod 24 is connected between the end of the armature and the center of the diaphragm 6. The base 4 is provided with a central aperture to accommodate the drive rod 24 and is suitably perforated as indicated at 2B in the conventional manner. The magnets, pole pieces and armature are conveniently secured together and to the base by a single fastening means, shown as a rivet 28. The unit may be provided with a suitable cover which is not shown in the drawing.

In operation as a sound-powered unit, the transducer may be used either as a receiver or a transmitter. For example, as a receiver upon application of a signal current to coil 22, a force is applied to flex the armature, decreasing one of the air gaps and increasing the other, and thereby moving the diaphragm to emit acoustic energy. Conversely, when the armature is flexed by acoustic energy impressed on the diaphragm, a signal voltage is generated in the coil 22.

The equivalent electric circuit of the system is shown in Fig. 3 wherein the batteries E are analogous to the magnetomotive forces of the magnets and the resistances Rm, Rp and Ra are analogous respectively to the reluctances of the magnet, the pole pieces, and the armature. The reluctances of the gaps l8 and 20 are represented by the resistances Rgl and RgZ. The flexing of the armature is analogous to the moving of a tap over these electrical resistances; thus, if the tap is moved upwardly Rgl increases and R-g2 decreases, the sum of Rgl and Rgfl remaining con-- stant. Thus motion of the tap up and down at a certain frequency will result in the flow of current at that frequency through the resistor R-a. Analogously, motion of the diaphragm sets up an alternating flux in the armature l8.

Assuming Rp and Ra negiligible, as is reasonable in practice, it can be shown that for a given magnetomotive force N1 in the coil, the force on the armature is proportional to (where Rg stands for the reluctance of either gap for the centralized position of the armature).

A comparative analysis for the conventional reed sound-powered unit may be made. The comparison is made on the basis that the conventional unit has the same air-gap polarizing flux and the same area of air gap at each end of the armature as at the free end of the armature of the present invention. This basis of comparison is necessarily dictated by the fact that the mechanical forces on the armature due to the polarizing flux, when the armature is 01f center, must in each case bear a similar relation to the mechanical stiffness of the armature, so that the armature will not be flexed into contact with one or the other of the pole tips. This analysis shows that the force due to a magnetomotive force NI in the coil is proportional to 2R, g+ m The quantity Rm is the incremental reluctance of the magnet, namely, the reluctance to alternating flux as indicated by the minor hysteresis loop for the magnet material. The magnets may be constructed of a material such as Alnico II which has a relatively high incremental permeability and a sufficiently high "energy product to provide the necessary polarizing flux with a small volume of material and an area of section. large relative to length. If Rm were negligible as compared with Rg, the comparative figure of merit would equal 2. Rm cannot be considered negligible, and this fact will reduce the figure of merit to about 1.4 or 1.5 for Alnico II, but in spite of this factor the present invention is seen to have a considerably higher electro-mechanical efficiency than conventional units.

In a system of transmitter and receiver, the figure of merit is the square of the figure of merit for a single unit.

The magnetic efficiency is also increased over the conventional reed arrangement. Since a complete magnetic circuit in the present invention includes only one air gap in which the polarizing flux must be maintained, the crosssectional area of the magnet material may be reduced. On a comparative basis it may be shown that the volume of magnet material required is less by to than in the conventional reed type.

The transducer is of simple and economical construction. For these reasons it is particularly suitable for very low cost telephones. It also recommends itself for other applications, where its high efiiciency is important, such as loudspeakers and high quality telephones. Although the apparatus is of particular utility in soundpowered or batteryless systems, it will be understood that it is equally applicable to units and systems in which direct current is used. The direct current may be isolated from the coil by a condenser, although the condenser is unnecessary if the resistance of the coil is sufficiently high. Thusthe transducer of the present invention may be used as a transmitter or receiver in a battery telephone system, or as a high-eniciency loudspeaker especially suitable for small radios; in any of such instances the device commends itself because of its compact construction, high efficiency and low cost.

Having thus described the invention, I claim:

1. A transducer comprising only two blocks of permanent magnet material magnetized as magnetically equivalent magnets in series, an armature comprising a straight strip secured at one end between the magnets and extending away therefrom for a substantial distance, two pole pieces attached to th outer faces of said magnets and extending therefrom in a direction generally parallel to the armature and being formed at their ends to approach the armature near the free end thereof, to form symmetrical magnetic circuits which are similar for both polarizing and alternating flux, the pole pieces being of low reluctance compared to the reluctance of the magnet material, means for rigidly securing the magnets, the armature and the pole pieces together, and a coil surrounding the armature and enclosed within the pole pieces.

2. A transducer comprising only two blocks of permanent magnet material magnetized as magnetically equivalent magnets in series, each block having an opening, an armature comprising a straight strip secured at one end between the magnets and extending away therefrom for a substantial distance, two pole pieces attached to the outer faces of said magnets and extending therefrom in a direction generally parallel to the armature and being formed at their ends to approach the armature near the free end thereof, to form symmetrical magnetic circuits which are similar for both polarizing and alternating flux, the pole pieces being of low reluctance compared to the reluctance of the magnet material, the armature and pole pieces having openings registering with the openings in the magnets, a fastener passed through all of said openings for rigidly securing the magnets, the armature and the pole pieces together, and a coil surrounding the armature and enclosed within the pole pieces.

WILLIAM J. MULDOON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,491,208 Peterson Apr. 22, 1924 2,163,161 Wadsworth June 20, 1939 2,215,782 Ellis Sept. 24, 1940 2,245,511 Turnbutt et al June 10, 1941 2,360,796 Roberton Oct. 17, 1944 2,381,673 Lehde Aug. 7, 1945 2,391,627 Howell Dec. 25, 1945 2,418,953 Rait Apr. 15, 1947 

